This invention generally relates to a light emitting device, and more specifically to an organic electroluminescent device of a blue luminescent material having an isoindolo [2,1-a] indole skeleton and good light-emitting properties.
The organic electroluminescent device (organic EL device) is a light emitting device, containing a fluorescent material which emits light in response to the recombination of hole and electron injected from anode and cathode (C. W. Tang et al. Applied Physics Letters, 51,913 (1987)). Luminescence efficiency can be improved through a method of doping a fluorescent dye. An organic EL device with a coumarin dye as the doping material (Applied Physics Letters, 65,3610 (1989)) can be used to greatly improve the luminescence efficiency. A C-545T (U.S. Pat. No. 4,769,292), which is a well-known coumarin dye, has the following structure: 
For improving the recombination efficiency of the injected hole and electron, multi-layered devices have been introduced. A hole transporting layer (HTL) containing hole transporting material (HTM) is used to improve the hole injection and transporting from the anode into the organic layer. An NPB (4-4xe2x80x2-bis[N-(1-naphthyl)-N-phenyl-amino-]bisphenyl), which is a well-known HTM, has the following structure: 
An electron transporting layer (ETL) consisting of an electron transporting material (ETM) is used to improve the electron injection from the cathode into the organic layer. An Alq3 (aluminum tris(8-hydroxyquinolate)), which is a typical ETM, has the following structure: 
Other materials such as oxadiazole compounds, triazine compounds and triazole compounds also can be used as ETM.
Aromatic dimethylidyne compounds have been used as the blue light emissive material for the organic EL device (U.S. Pat. No. 6,093,864). One example is DPVBi (1,4-bis(2,2-di-phenylvinyl)) biphenyl with an EL peak at about 485 nm, having the following structure: 
Other blue light emissive materials for organic EL device include:
1,3-Dibenzyldieneindane compound(U.S. Pat. No. 6,180,267), an example of them has the following structure: 
o-(N-aryl-2-benzimidazolyl)phenol organometallic complex (U.S. Pat. No. 5,755,999), an example of them has the following structure: 
wherein M is a divalent or a trivalent metal.
Heterocyclic compounds (JP-2000-299186, JP-2001-23777, JP-2001-35664, JP-2001-118683, JP-2001-196181, JP-2001-196182), some examples of these compounds are shown with the following structures: 
An object of the present invention is to provide a material having an isoindolo [2,1-a] indole skeleton and to provide an organic EL device having blue luminescence. The organic EL device comprises an anode, cathode, and one or more organic thin film layers which contain, either singly or as a mixture, an indole compound represented by the following general formula (1): 
wherein Ar1 represents a substituted or unsubstituted aromatic hydrocarbon group or a substituted or unsubstituted aromatic heterocyclic group. X represents a single bond or xe2x80x94CHxe2x95x90CHxe2x80x94 group. R1 to R8 each independently represents a hydrogen atom, a halogen atom, a nitro group, a cyano group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted amino group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkoxycarbonyl group, a substituted or unsubstituted aromatic hydrocarbon group or a substituted or unsubstituted aromatic heterocyclic group. Any two of R1 to R8 may form a ring.